config: remove HAVE_KTIME_GET_RAW_TS64

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16479

include/os/linux/spl/sys/time.h

@@ -81,13 +81,8 @@ gethrestime_sec(void)
 static inline hrtime_t
 gethrtime(void)
 {
-#if defined(HAVE_KTIME_GET_RAW_TS64)
 	struct timespec64 ts;
 	ktime_get_raw_ts64(&ts);
-#else
-	struct timespec ts;
-	getrawmonotonic(&ts);
-#endif
 	return (((hrtime_t)ts.tv_sec * NSEC_PER_SEC) + ts.tv_nsec);
 }